301 results on '"Jakob Birkedal Wagner"'
Search Results
152. (Invited) In Situ TEM Study of Reduction and Reoxidation of NiO and NiO-YSZ Composites
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Jan Van herle, Quentin Jeangros, Jakob Birkedal Wagner, Thomas Willum Hansen, Aïcha Hessler-Wyser, Cécile Hébert, and Rafal Dunin-Borkowski
- Abstract
Nickel-yttria-stabilised zirconia (YSZ) solid oxide fuel cell (SOFC) anodes exhibit a dimensional instability when experiencing a reduction-oxidation cycle. As fuel is supplied on the anode side, as-sintered NiO reduces to metallic Ni and remains in this state during operation; yet several factors may lead to an accidental reoxidation of the Ni, which may rupture parts of the cell, hence degrading its performance. The mechanisms behind the dimensional instability of Ni(-YSZ) anodes were investigated through environmental transmission electron microscopy. NiO particles and NiO/YSZ composites were reduced and reoxidised in the microscope in a few mbar of H2 and O2, respectively, up to 500-850°C. Images, diffraction patterns, electron energy-loss spectra (EELS) and energy-filtered micrographs (EFTEM) were acquired to capture in situ the nanostructure, crystallography and chemistry. Reaction kinetics were retrieved from both the changes in shapes of the Ni L2,3 edges in energy-loss spectra and from energy-filtered images (with nm-resolution), to provide quantitative data, which were correlated to the nanostructure. Figure – (a) as-sintered NiO particles observed (b) after reduction (at 600°C during 210 min in 1.3 mbar of H2) and (c) after reoxidation (at 600°C) after heating from 250°XC at a rate of 4°C/min in 3.2 mbar of O2). In (d), the projected areas of (a–c) are superimposed to the image shown in (c). Reduction results in a shrinkage of the projected area of -15% between (a) and (b) in the region of interest circled in (d), while oxidation induces an expansion of the projected area of +65% between (b) and (c). The projected area of NiO expands by +40% between (a) and (c) after one redox cycle. While the surface nucleation of Ni domains, their growth and impingement control the reduction of NiO particles (Avrami-type mechanism), the results revealed a modification of the mechanisms in the presence of YSZ, with the transfer of oxygen from NiO to the oxygen vacancies of YSZ triggering the reaction, before free surface reduction starts. Intragranular voids formed in both cases as oxygen is removed. The final Ni structure at high temperature was then observed to coarsen as it minimises its surface energy, with the percolation of the Ni phase influenced by the symmetry of its grain boundaries. The reoxidation of Ni is controlled mainly by the outward diffusion of Ni ions through the grain boundaries of the growing NiO film, rather than by a Ni/NiO or NiO/O2 interfacial reaction limitation. While some NiO inward growth occurred through the formation of oxide film cracks, the Ni2+ outward diffusion process remains unbalanced and voids form in the NiO phase. These internal voids are responsible for the dimensional instability of the composite along with Ni coarsening at high temperature. Several parameters for improved performance and redox tolerance are identified based on these results. Complementary data included post-exposure microscopy, in situ X-ray diffraction and density functional theory computations. With the complementary XRD results and analyses done on larger sample volumes, larger H2 gas flow and pressure than in the E-TEM conditions, it was shown that the reduction reaction in the ETEM is intrinsic, up to a conversion plateau of 60%, after which it is limited by too slow evacuation of the steam produced by the reduction reaction itself. Scanning TEM (STEM) with elemental analysis (EDX) was employed for trace impurity detection in NiO-YSZ structures, both for as sintered and exposed samples, showing the segregation of impurities (mainly Al and Si) at Ni-Ni interfaces in the form of a glassy film supporting Ni nanoparticles. Some of these impurities were unexpected and could affect the anode performance during operation. Figure 1
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- 2016
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153. Nickel oxide reduction studied by environmental TEM and in situ XRD
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Jeangros, Q., Thomas Willum Hansen, Jakob Birkedal Wagner, Christian Danvad Damsgaard, Dunin-Borkowski, Rafal E., Hébert, C., Herle, J., and Hessler-Wyser, A.
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Nickel oxide reduction and subsequent Ni behaviour under H2 is of practical importance in the field of solid oxide fuel cell (SOFC) technology, as it determines the structure of the electronic conductor on the anode side during operation [1]. Despite extensive coverage in the literature [2], several questions remain, notably regarding the NiO reduction kinetics and its structural evolution.Here, we perform in situ reduction of an industrial NiO powder from JT Baker™ under 1.3 mbar of H2 (2 mlN/min) in a differentially pumped FEI Titan environmental transmission electron microscope (ETEM) [3]. Images, diffraction patterns and electron energy-loss spectra are acquired during different temperature ramps (300 to 600°C) in order to monitor the structural and chemical evolution of the system. Similar experiments are performed in a PANalytical X'Pert PRO X-ray diffractometer equipped with an Anton Paar furnace and a mass spectrometer to complement and extrapolate the ETEM data to higher pressures and gas flows. Diffraction peak intensities, crystallite sizes (obtained using the Scherrer formula) and H2/H2O mass spectrometer signals are monitored.The recorded images and diffraction patterns show that the nucleation of Ni on NiO takes place at temperatures below 400°C and is either epitaxial with a small angle misfit in thin specimen areas (Fig. 1) or randomly oriented in thicker regions. Reduction proceeds through the growth of Ni domains and the movement of interfaces, the latter inducing particle shrinkage and pore creation within larger particles. Densification is observed when the temperature approaches 600°C: the porosity that was created at lower temperatures disappears. This reorganization of Ni is detrimental to the connectivity of the Ni catalyst and to the reduction-reoxidation stability of the SOFC [4].Reaction kinetics are measured in the ETEM by electron energy-loss spectroscopy using two different techniques based on changes of the Ni L2,3 white lines upon reduction (either taking the ratio of the Ni L3 and L2 integrated intensities or using multiple linear least squares fitting of NiO and Ni experimental reference spectra [5,6]). Reduction starts earlier but at a higher temperature when the heating rate is increased (380°C at 2°C/min compared to 420°C at 7°C/min). The initiation of the reduction depends on the number of oxygen surface vacancies, which in turn depends on the temperature and time [7]. Reduction proceeds quickly until the remaining NiO fraction reaches a level of about 30% (Fig. 2). The reaction then slows down and full reduction is only achieved at temperatures above 600°C (confirmed using diffraction). This decrease in reaction rate appears to be caused by physical factors, as NiO reduction is known to be autocatalytic [2]. As only a few micrograms of reducible material are dispersed on the TEM grid and considering the volume of the reaction chamber in the ETEM (~800 cm3), a shortage of reactive gas is unlikely even in 2 mbar of H2. It is probable that the presence and stagnation of H2O vapour created upon reduction blocks H2 access to the reactive sites, decreasing the reaction rate at a high conversion fraction. The gas flow is low and may not evacuate the product gas efficiently around the Ni(O) particles. Complementary XRD experiments point towards the same conclusion and underline the impact of the pressure and gas flow on the kinetics.It is possible to determine an approximate activation energy for NiO reduction inside the ETEM using the Kissinger method [8], which is based on measurements of the shift in the temperature at which the reaction rate is maximum for different heating rates. With EELS experiments performed at 2, 4 and 7 °C/min, an activation energy of ~70 ± 20 kJ/mol is obtained, which is consistent with values found in the literature [2].
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- 2012
154. Environmental Transmission Electron Microscopy in an Aberration-Corrected Environment
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Jakob Birkedal Wagner and Thomas Willum Hansen
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Conventional transmission electron microscope ,Microscope ,Chemistry ,business.industry ,Aberration correction ,Catalysis ,law.invention ,Scattering ,Optics ,Electron tomography ,Transmission electron microscopy ,law ,Environmental Transmission Electron Microscope ,Scanning transmission electron microscopy ,Energy filtered transmission electron microscopy ,Environmental transmission electron microscopy (ETEM) ,business ,High-resolution transmission electron microscopy ,Instrumentation - Abstract
The increasing use of environmental transmission electron microscopy (ETEM) in materials science provides exciting new possibilities for investigating chemical reactions and understanding both the interaction of fast electrons with gas molecules and the effect of the presence of gas on high-resolution imaging. A gaseous atmosphere in the pole-piece gap of the objective lens of the microscope alters both the incoming electron wave prior to interaction with the sample and the outgoing wave below the sample. Whereas conventional TEM samples are usually thin (below 100 nm), the gas in the environmental cell fills the entire gap between the pole pieces and is thus not spatially localized. By using an FEI Titan environmental transmission electron microscope equipped with a monochromator and an aberration corrector on the objective lens, we have investigated the effects on imaging and spectroscopy caused by the presence of the gas.
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- 2012
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155. ETEM study of deactivation of NiGa nanoparticles as catalyst for methanol synthesis
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Christian Danvad Damsgaard, Linus Daniel Leonhard Duchstein, Diego Gardini, Irek Sharafutdinov, Christian Fink Elkjær, Søren Dahl, and Jakob Birkedal Wagner
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- 2012
156. Nickel oxide reduction studied by environmental TEM
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Jeangros, Q., Thomas Willum Hansen, Jakob Birkedal Wagner, Christian Danvad Damsgaard, Dunin-Borkowski, R. E., Hébert, C., Herle, J., and Hessler-Wyser, A.
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In situ reduction of an industrial NiO powder is performed under 1.3 mbar of H2 (2 mlN/min) in a differentially pumped FEI Titan 80-300 environmental transmission electron microscope (ETEM). Images, diffraction patterns and electron energy loss spectra (EELS) are acquired to monitor the structural and chemical evolution of the system during different temperature ramps (at 2, 4 and 7°C/min). High-resolution ETEM is also performed during similar experiments.Ni nucleation on NiO is observed to be either epitaxial in thin areas or randomly oriented on thicker regions and when nucleation is more advanced. The growth of Ni crystallites and the movement of interfaces create pores within the NiO grains to accommodate the volume shrinkage associated with the reduction. EELS analysis illustrates that reduction proceeds quickly at temperatures below 400°C up to a reduced fraction of about 0.6, until the reaction is slowed down by water created upon reduction. Using the data obtained at different heating rates and the Kissinger method, an activation energy for the NiO reduction of 70 ± 20 kJ/mol could be obtained. Densification is then observed at temperatures higher than 550°C: pores created at lower temperatures disappear and Ni grains coarsen. This reorganization of Ni is detrimental to both the connectivity of the Ni catalyst and to the redox stability of the SOFC. A model for the structural evolution of NiO under H2 is proposed.
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- 2012
157. Atom-Resolved Imaging of Dynamic Shape Changes in Supported Copper Nanocrystals
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Poul L. Hansen, Bjerne S. Clausen, Jakob Birkedal Wagner, Jens R. Rostrup-Nielsen, Stig Helveg, and Henrik Topsøe
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inorganic chemicals ,Multidisciplinary ,Nanoparticle ,chemistry.chemical_element ,Nanotechnology ,Copper ,Surface energy ,Catalysis ,Nanomaterials ,Chemical engineering ,Transition metal ,Nanocrystal ,chemistry ,Transmission electron microscopy ,sense organs ,skin and connective tissue diseases - Abstract
In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide–supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.
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- 2002
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158. Nanometer-scale lithography on microscopically clean graphene
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Ben L. Feringa, Thomas Willum Hansen, Jakob Birkedal Wagner, W. F. van Dorp, Xiaoyan Zhang, J.Th.M. De Hosson, and Stratingh Institute of Chemistry
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Materials science ,Graphene ,Mechanical Engineering ,Graphene foam ,FABRICATION ,Bioengineering ,Nanotechnology ,General Chemistry ,Substrate (electronics) ,MEMBRANES ,FILMS ,law.invention ,Amorphous solid ,Mechanics of Materials ,law ,Deposition (phase transition) ,General Materials Science ,INDUCED DEPOSITION ,Electrical and Electronic Engineering ,Lithography ,FOCUSED ELECTRON-BEAM ,TEMPERATURE ,Graphene nanoribbons ,Graphene oxide paper - Abstract
Focused-electron-beam-induced deposition, or FEBID, enables the fabrication of patterns with sub-10 nm resolution. The initial stages of metal deposition by FEBID are still not fundamentally well understood. For these investigations, graphene, a one-atom-thick sheet of carbon atoms in a hexagonal lattice, is ideal as the substrate for FEBID writing. In this paper, we have used exfoliated few-layer graphene as a support to study the early growth phase of focused-electron-beam-induced deposition and to write patterns with dimensions between 0.6 and 5 nm. The results obtained here are compared to the deposition behavior on amorphous materials.Prior to the deposition experiment, the few-layer graphene was cleaned. Typically, it is observed in electron microscope images that areas of microscopically clean graphene are surrounded by areas with amorphous material. We present a method to remove the amorphous material in order to obtain large areas of microscopically clean graphene flakes. After cleaning, W(CO) 6 was used as the precursor to study the early growth phase of FEBID deposits. It was observed that preferential adsorption of the precursor molecules on step edges and adsorbates plays a key role in the deposition on cleaned few-layer graphene.
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- 2011
159. Discrete dynamics of nanoparticle channelling in suspended graphene
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Timothy J. Booth, Peter Bøggild, Thomas Willum Hansen, Jakob Birkedal Wagner, Ole Hansen, Rafal E. Dunin-Borkowski, Filippo Pizzocchero, Joerg R. Jinschek, and Henrik Andersen
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Materials science ,Silver ,Surface Properties ,Nanoparticle ,Metal Nanoparticles ,Bioengineering ,Nanotechnology ,Activation energy ,Channelling ,Silver nanoparticle ,law.invention ,Microscopy, Electron, Transmission ,law ,Environmental Transmission Electron Microscope ,General Materials Science ,Particle Size ,Nanoscopic scale ,Graphene ,Mechanical Engineering ,General Chemistry ,Condensed Matter Physics ,Chemical physics ,Transmission electron microscopy ,Thermodynamics ,Graphite ,Oxidation-Reduction - Abstract
We have observed a previously undescribed stepwise oxidation of mono- and few layer suspended graphene by silver nanoparticles in situ at subnanometer scale in an environmental transmission electron microscope. Over the range of 600-850 K, we observe crystallographically oriented channelling with rates in the range 0.01-1 nm/s and calculate an activation energy of 0.557 ± 0.016 eV. We present a discrete statistical model for this process and discuss the implications for accurate nanoscale patterning of nanoscale systems.
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- 2011
160. Development of a versatile TEM specimen holder for the characterization of photocatalytic materials
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Filippo Cavalca, Langhammer, C., Thomas Willum Hansen, Jakob Birkedal Wagner, and Dunin-Borkowski, Rafal E.
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Nanoplasmonic sensing ,TEM ,In situ light - Abstract
Photocatalysts are of fundamental interest for sustainable energy research [1]. By means of transmission electron microscopy (TEM), it is possible to obtain insight into their structure, composition and reactivity. Such insight can then be used for their further optimization [2]. Here, we combine conventional TEM analysis on photocatalysts with several in situ TEM techniques including environmental TEM (ETEM), in situ photo activation and localized surface plasmon resonance (LSPR) spectroscopy [3-4]. These experiments are facilitated by the construction of a specimen holder capable of illuminating samples inside the TEM with a laser diode and an optical system to guide light onto the sample with maximum power transmission. The source can be changed and tuned between the visible and the UV range. The specimen holder is equipped with five electrical contacts that can be used to perform in situ electrical, pressure and temperature measurements as well as to power custom MEMS-based in situ heaters. It can be used inside an ETEM allowing specimens to be analysed in a controlled gas atmosphere during biasing and illuminating experiments. A second, more advanced version of the holder has also been developed allowing simultaneous illumination of a specimen by several light sources using a combination of up to three monochromatic wavelengths and/or broadband light. Light sources and combinations can be changed during operation without interfering with the electron microscope operation. An additional optical probe provides the ability to collect light emitted from the sample and to perform spectroscopic analysis in real time, in parallel with all other in situ techniques and analytical capabilities offered by the specimen holder and the microscope. The holders are presently being used to study a variety of photoreactive materials and structures, including photocatalysts (Fig. 1), photonic devices and solar cells with a particular focus on LSPR spectroscopy performed by indirect nanoplasmonic sensing [3-4], a novel experimental platform for measurements of thermodynamics and kinetics in/on nanomaterials and thin films. We present results from combined ETEM-LSPR studies of thermally-induced catalytic phenomena and on metal nanoparticle sintering, an important process in catalysis, which we study in situ in both real and model systems by means of indirect nanoplasmonic sensing and ETEM.
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- 2011
161. Nano-Schottky contacts realized by bottom-up technique
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Maria E. Messing, Jakob Birkedal Wagner, Lars Samuelson, Lars Montelius, Dmitry Suyatin, Håkan Pettersson, and Johanna Trägårdh
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III-V semiconductors ,Materials science ,Nanostructure ,Nanowire ,Photodetector ,Nanotechnology ,Epitaxy ,Nano ,Fysik ,catalysis ,business.industry ,epitaxial growth ,semiconductor quantum wires ,Schottky diode ,Heterojunction ,semiconductor epitaxial layers ,Schottky barriers ,semiconductor growth ,Characterization (materials science) ,indium compounds ,nanowires ,Physical Sciences ,photodetectors ,Optoelectronics ,gallium compounds ,business - Abstract
Properties of nanostructures realized by bottom-up techniques are often different from their bulk counterparts.1 Here we present a study of a nano-Schottky contact formed at the interface between a gold catalytic particle and an epitaxially grown Ga x In 1−x As/InAs nanowire. Selective electrical connections formed to the catalytic particle on one side and to the InAs segment on the other side allowed electrical and optical characterization of the formed junction. We demonstrate that the heterostructure region adjacent to the catalytic particle may act as an ultra-small volume unipolar photodetector with potentially ultra-fast response.
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- 2010
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162. Microphotoluminescence studies of tunable wurtziteInAs0.85P0.15quantum dots embedded in wurtzite InP nanowires
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Lisa Karlsson, Kimberly A. Dick, Jakob Birkedal Wagner, Niklas Sköld, Mats-Erik Pistol, Craig Pryor, and Lars Samuelson
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Condensed Matter::Materials Science ,Materials science ,Effective mass (solid-state physics) ,Condensed matter physics ,Quantum dot ,Band gap ,Nanowire ,Electron ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Spectroscopy ,Electronic, Optical and Magnetic Materials ,Wurtzite crystal structure - Abstract
We have investigated the effects of strong confinement in wurtzite InAs0.85P0.15 quantum dots embedded in wurtzite InP nanowires using microphotoluminescence spectroscopy. Strain-dependent k . p calculations were used to model the quantum dots, and it was found that the electron effective mass was increased by a factor of 2 and the band gap was increased by 190 meV compared to equivalent quantum dots in the zinc-blende polytype. Measurements indicate that there is a relaxation bottleneck giving rise to an anomalous state filling behavior.
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- 2009
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163. Giant, level-dependent g factors in InSb nanowire quantum dots
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Henrik Nilsson, Jakob Birkedal Wagner, Hongqi Xu, Claes Thelander, Lars Samuelson, Philippe Caroff, Marcus Larsson, Lars-Erik Wernersson, Institut d’Électronique, de Microélectronique et de Nanotechnologie - UMR 8520 (IEMN), and Centrale Lille-Institut supérieur de l'électronique et du numérique (ISEN)-Université de Valenciennes et du Hainaut-Cambrésis (UVHC)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Université Polytechnique Hauts-de-France (UPHF)
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Antimony ,Silicon ,Surface Properties ,Quantum point contact ,Nanowire ,Electrons ,Bioengineering ,02 engineering and technology ,Indium ,01 natural sciences ,Condensed Matter::Materials Science ,Materials Testing ,Quantum Dots ,0103 physical sciences ,Principal quantum number ,Nanotechnology ,General Materials Science ,Particle Size ,010306 general physics ,Quantum ,Physics ,Condensed matter physics ,Nanowires ,Mechanical Engineering ,General Chemistry ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Semiconductors ,Quantum dot laser ,Quantum dot ,Excited state ,Quantum Theory ,0210 nano-technology ,Ground state - Abstract
We report on magnetotransport measurements on InSb nanowire quantum dots. The measurements show that the quantum levels of the InSb quantum dots have giant g factors, with absolute values up to approximately 70, the largest value ever reported for semiconductor quantum dots. We also observe that the values of these g factors are quantum level dependent and can differ strongly between different quantum levels. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of Delta(SO) = 280 mueV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons.
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- 2009
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164. Growth of vertical InAs nanowires on heterostructured substrates
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Francesca Rossi, Linus Fröberg, Philippe Caroff, Lars-Erik Wernersson, Jakob Birkedal Wagner, Kristian Nilsson, Lucia Sorba, Giorgio Biasiol, Lars Samuelson, Claudio Bocchi, Stefano Roddaro, Institut d’Électronique, de Microélectronique et de Nanotechnologie - UMR 8520 (IEMN), and Centrale Lille-Institut supérieur de l'électronique et du numérique (ISEN)-Université de Valenciennes et du Hainaut-Cambrésis (UVHC)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Université Polytechnique Hauts-de-France (UPHF)
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Nanostructure ,Materials science ,growth ,Nanowire ,Bioengineering ,Gallium ,02 engineering and technology ,Epitaxy ,Microscopy, Atomic Force ,01 natural sciences ,Indium ,Arsenicals ,Microscopy, Electron, Transmission ,InAs ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,010302 applied physics ,business.industry ,Nanowires ,Mechanical Engineering ,Heterojunction ,heterostructure ,General Chemistry ,021001 nanoscience & nanotechnology ,Semiconductor ,Mechanics of Materials ,Transmission electron microscopy ,Optoelectronics ,0210 nano-technology ,business ,Layer (electronics) ,Molecular beam epitaxy - Abstract
We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 μm thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.
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- 2009
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165. Chemical composition and morphology of individual aerosol particles from a CARIBIC flight at 10 km altitude between 50°N and 30°S
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Jost Heintzenberg, E. Carlemalm, Stephan Weinbruch, Carl A. M. Brenninkmeijer, Tanja Schuck, Andreas Zahn, Jakob Birkedal Wagner, Bengt G. Martinsson, P. F. J. van Velthoven, Hung N. Nguyen, M. Hermann, and Martin Ebert
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Atmospheric Science ,Ecology ,Paleontology ,Soil Science ,Single particle analysis ,Mineralogy ,Forestry ,Aquatic Science ,Oceanography ,Atmospheric sciences ,Aerosol ,Troposphere ,Atmosphere ,Geophysics ,Altitude ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Particle ,Environmental science ,Southern Hemisphere ,Stratosphere ,Earth-Surface Processes ,Water Science and Technology - Abstract
Analysis of individual particles by analytical electron microscopy as well as quantitative analysis using particle-induced X-ray emission (PIXE) and particle elastic scattering analysis (PESA) were carried out on samples collected from a flight at 10 km altitude between 50 degrees N and 30 degrees S as part of the Civil Aircraft for Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) project (http://www.caribic-atmospheric.com). Particle morphology showed large variability with sampling latitude. Complicated branched structures dominated the large particles of the extratropical northern and southern hemisphere and the northern tropics. Particles in the tropics of the southern hemisphere were small in size and large in number concentration, whereas particles in or close to the intertropical convergence zone were few and small in size. Particles in the lowermost stratosphere were found to have similar structures but more branched than the ones found in the upper troposphere of the extratropics. Quantitative analysis revealed that the sulfur concentration varied by a factor of 50 in the nine samples analyzed in this study. The carbon-to-sulfur mass concentration ratio was lowest in the lowermost stratosphere (0.5) and highest in the tropics of the southern hemisphere (3.5). The elemental distribution of carbon and sulfur in individual particles was mapped by energy-filtered transmission electron microscopy (EFTEM). Almost all particles analyzed contained a mixture of carbonaceous and sulfurous matter. Particles with satellites were found by EFTEM to contain both carbon and sulfur in the central particle, whereas in the satellite particles only carbonaceous material was detected. (Less)
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- 2008
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166. Epitaxial integration of nanowires in microsystems by local micrometer-scale vapor-phase epitaxy
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Dirch Hjorth Petersen, Peter Bøggild, Lars Samuelson, Brent A. Wacaser, Kristian Mølhave, and Jakob Birkedal Wagner
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Silicon ,Materials science ,Nanowires ,Nanowire ,Temperature ,chemistry.chemical_element ,Nanotechnology ,General Chemistry ,Chemical vapor deposition ,Epitaxy ,Biomaterials ,chemistry ,Electricity ,Microsystem ,General Materials Science ,Metalorganic vapour phase epitaxy ,Gases ,Vapor–liquid–solid method ,Volatilization ,Crystallization ,Biotechnology ,Microfabrication - Abstract
Free-standing epitaxially grown nanowires provide a controlled growth system and an optimal interface to the underlying substrate for advanced optical, electrical, and mechanical nanowire device connections. Nanowires can be grown by vapor-phase epitaxy (VPE) methods such as chemical vapor deposition (CVD) or metal organic VPE (MOVPE). However, VPE of semiconducting nanowires is not compatible with several microfabrication processes due to the high synthesis temperatures and issues such as cross-contamination interfering with the intended microsystem or the VPE process. By selectively heating a small microfabricated heater, growth of nanowires can be achieved locally without heating the entire microsystem, thereby reducing the compatibility problems. The first demonstration of epitaxial growth of silicon nanowires by this method is presented and shows that the microsystem can be used for rapid optimization of VPE conditions. The important issue of the cross-contamination of other parts of the microsystem caused by the local growth of nanowires is also investigated by growth of GaN near previously grown silicon nanowires. The design of the cantilever heaters makes it possible to study the grown nanowires with a transmission electron microscope without sample preparation.
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- 2008
167. A radio frequency single-electron transistor based on an InAs/InP heterostructure nanowire
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Christopher Wilson, Simon Abay, Claes Thelander, Per Delsing, Jakob Birkedal Wagner, Henrik Nilsson, Tim Duty, and Lars Samuelson
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Materials science ,business.industry ,Mechanical Engineering ,Transistor ,Nanowire ,Coulomb blockade ,Bioengineering ,Nanotechnology ,Heterojunction ,General Chemistry ,Condensed Matter Physics ,law.invention ,chemistry.chemical_compound ,chemistry ,law ,Indium phosphide ,Optoelectronics ,General Materials Science ,Electrical measurements ,Radio frequency ,business ,Noise (radio) - Abstract
We demonstrate radio frequency single-electron transistors fabricated from epitaxially grown InAs/InP heterostructure nanowires. Two sets of double-barrier wires with different barrier thicknesses were grown. The wires were suspended 15 nm above a metal gate electrode. Electrical measurements on a high-resistance nanowire showed regularly spaced Coulomb oscillations at a gate voltage from -0.5 to at least 1.8 V. The charge sensitivity was measured to 32 microe rms Hz(-1/2) at 1.5 K. A low-resistance single-electron transistor showed regularly spaced oscillations only in a small gate-voltage region just before carrier depletion. This device had a charge sensitivity of 2.5 microe rms Hz(-1/2). At low frequencies this device showed a typical 1/f noise behavior, with a level extrapolated to 300 microe rms Hz(-1/2) at 10 Hz.
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- 2008
168. Optical properties of rotationally twinned InP nanowire heterostructures
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Thomas Mårtensson, Jiming Bao, Federico Capasso, Jakob Birkedal Wagner, David C. Bell, Lars Samuelson, and Johanna Trägårdh
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Photoluminescence ,Materials science ,Condensed matter physics ,business.industry ,Mechanical Engineering ,Nanowire ,Physics::Optics ,Bioengineering ,Heterojunction ,General Chemistry ,Electronic structure ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Condensed Matter::Materials Science ,Semiconductor ,Band diagram ,General Materials Science ,Crystal twinning ,business ,Electronic band structure - Abstract
We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.
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- 2008
169. Substituted Mo-V(Ti)-Te(Ce)-oxide M2 Catalysts for Propene Ammoxidation
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Johan Holmberg, Jean-Marc M. Millet, Jakob Birkedal Wagner, Robert K. Grasselli, Benoit Deniau, Robert Häggblad, Arne Andersson, Staffan Hansen, Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON), Université Claude Bernard Lyon 1 (UCBL), and Université de Lyon-Université de Lyon-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)
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Materials science ,010405 organic chemistry ,Inorganic chemistry ,General Chemistry ,[CHIM.CATA]Chemical Sciences/Catalysis ,010402 general chemistry ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Propene ,chemistry.chemical_compound ,Crystallography ,chemistry ,Phase (matter) ,Orthorhombic crystal system ,Acrylonitrile ,Selectivity ,High-resolution transmission electron microscopy ,Ammoxidation - Abstract
Haggblad, Robert Wagner, Jakob B. Deniau, Benoit Millet, Jean-Marc M. Holmberg, Johan Grasselli, Robert K. Hansen, Staffan Andersson, Arne 4th Irsee Symposium 2007 JUN 07-10, 2007 Irsee, GERMANY; One of the most effective propane to acrylonitrile ammoxidation catalyst is comprised of the two phases M1 (orthorhombic) Mo7.5V1.5NbTeO29 and M2 (pseudo-hexagonal) Mo4V2Te2O20. Under reaction conditions, the two phases work in symbiosis with each other where M1 is the paraffin activating component and M2 is the olefin activating component. Since the catalytic improvement of either phase should result in an enhancement of the overall acrylonitrile yield, controlled substitution of certain elements in either or both phases might result in the desired improvement. Our current study concentrates on the partial substitutions of V with Ti and Te with Ce in the M2 phase. Ti substitution results in a considerable propene activity improvement, whereas the selectivity to acrylonitrile is unaffected. Substitution with Ce, on the contrary, substantially improves the selectivity to acrylonitrile. Also, a minor improvement of the activity is notable. The acrylonitrile selectivity improvement is a result of better NH3 utilization and comes at the expense of reduced acrolein make. XRD reveals that all of the substituted compositions retain the M2 structure and essentially are monophasic. XANES recordings show for the bulk that the Mo is 6+, the V is 4+, or 4+ and 5+ when Ce is present, the Ti is 4+, the Ce is 3+, and the Te 4+ with some 6+ also present. According to the ESR data, in the M2 with Ce (7Te/3Ce) only 21% of the V is 4+, the remainder being 5+, which tentatively can be explained by the existence of some cation vacancies in the hexagonal channels. HRTEM imaging reveals little if any differences between the materials, all have the typical pseudo-hexagonal habit of the M2 phase and expose a 1-2 nm thick surface layer without any apparent long-range ordering. XPS data show that all catalysts, including the base, are highly enriched at the surface with Te at the expense of other metals. The 7Te/3Ce composition exhibits also substantial Ce surface enrichment. Moreover, the valences of the cations at the surface differ from the bulk in that for all fresh catalysts V is 5+ and Te is 6+ on the surface. Characterization by XPS of catalysts used in propene ammoxidation, reveals reduction of Te and, except when Ce is present, also Mo. Therefore, it might be inferred that the surfaces of the catalysts studied here are comprised essentially of one or a few monolayers of TeMoO or TeCeMoO on an interacting M2 crystalline base.
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- 2008
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170. Increasing amperometric biosensor sensitivity by length fractionated single-walled carbon nanotubes
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Gilbert Nöll, Jakob Birkedal Wagner, Lo Gorton, and Federico Tasca
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Size-exclusion chromatography ,Biomedical Engineering ,Biophysics ,Analytical chemistry ,Carbon nanotube ,Biosensing Techniques ,Redox ,Sensitivity and Specificity ,law.invention ,Gel permeation chromatography ,law ,Electrochemistry ,Nanotechnology ,Particle Size ,Detection limit ,Chemistry ,Nanotubes, Carbon ,Reproducibility of Results ,General Medicine ,Equipment Design ,NAD ,Amperometry ,Equipment Failure Analysis ,Linear range ,Biosensor ,Biotechnology - Abstract
In this work the sensitivity-increasing effect of single-walled carbon nanotubes (SWCNTs) in amperometric biosensors, depending on their average length distribution, was studied. For this purpose the SWCNTs were oxidatively shortened and subsequently length separated by size exclusion chromatography. Transmission electron micrographs of different fractions of SWCNTs were collected. Diaphorase "wired" to an osmium redox polymer was blended with the shortened SWCNTs of different lengths. Depending on the average length of the SWCNTs the sensitivity of the amperometric biosensor model system towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH) was increased by a factor of five. The best performance was achieved with SWCNTs of medium length. The linear range for NADH detection was between 5 microM and 7 mM, the maximum sensitivity was 47 nA microM(-1) cm(-2), and the detection limit was 1 microM. The biosensor exhibited excellent electrocatalytic properties. Even at relatively high NADH concentrations the oxidative current was limited by the diffusion rate of NADH.
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- 2007
171. Cover Picture: Epitaxial Growth of Indium Arsenide Nanowires on Silicon Using Nucleation Templates Formed by Self-Assembled Organic Coatings (Adv. Mater. 14/2007)
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Edvin Lundgren, Julian Stangl, Thomas Mårtensson, B. J. Ohlsson, Emelie Hilner, Jakob Birkedal Wagner, Werner Seifert, Claes Thelander, Lars Samuelson, Anders Mikkelsen, and Anders Gustafsson
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Materials science ,Silicon ,Mechanical Engineering ,Nanowire ,Nucleation ,chemistry.chemical_element ,Nanotechnology ,Substrate (electronics) ,Epitaxy ,chemistry.chemical_compound ,chemistry ,Nanoelectronics ,Mechanics of Materials ,General Materials Science ,Indium arsenide ,Vapor–liquid–solid method - Abstract
On p. 1801, Lars Samuelson and co-workers report on InAs nanowires that are grown directly on Si substrates by employing self-assembled organic coatings to create an oxide template which guides nanowire nucleation. The nanowires extend vertically from the Si(111) substrate (foreground). No metal catalysts are used, and the InAs crystal extends to the nanowire tip as shown in the atomically resolved transmission electron microscopy image (dome background). The reported method constitutes a promising approach to the integration of new components into existing Si technology.
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- 2007
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172. The physicochemical properties of rutile-supported V-O-Mo catalyst
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Jakob Birkedal Wagner, Pawel Kornelak, Dang Sheng Su, Lidia Lityńska-Dobrzyńska, J. Camra, Aleksandra Wesełucha-Birczyńska, and B. Borzęcka-Prokop
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Chemistry ,XRD ,Analytical chemistry ,General Chemistry ,Heterogeneous catalysis ,Decomposition ,Catalysis ,Chemical engineering ,rutile-supported V-O-Mo catalyst ,Rutile ,X-ray crystallography ,Monolayer ,TEM ,sol-gel method ,Crystallite ,FT Raman spectroscopy ,Sol-gel - Abstract
This paper concerns the physicochemical properties and activity in NO decomposition of a rutile-supported V-O-Mo catalyst, obtained by sol–gel method. Rutile, o-MoO3 and β-MoO3 are the main phases found by XRD and TEM. Monolayer vanadia-like species containing some Mo atoms substituted for V ones are present on molybdena crystallites. These species show some activity in NO decomposition to N2 and O2. An increase in this activity with the increase of O2 content in the feed indicates surface reconstruction occurring due to interaction with oxygen.
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- 2007
173. (Invited) Structure and Stability of Pt-Y Alloy Particles for Oxygen Reduction Studied by Electron Microscopy
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Davide Deiana, Jakob Birkedal Wagner, and Thomas Willum Hansen
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Nanostructured catalysts play an increasingly large role in contemporary society and understanding their behavior in their working state has been primary area of focus for many research groups over the past decades. With dwindling supplies of the precious metals often used in these catalysts, their effective use becomes increasingly important. Recent developments in electron microscopy as well as computational modelling now allow us to gain new insight into the structure and stability of these catalysts in an environment resembling their working conditions. Such insights will enable scientists to effectively synthesize stable nanostructures with an increased performance and lifetime. For heterogeneous catalysts in the form of supported metal nanoparticles, surfaces play a crucial role. This is valid for both electrocatalysts as well as traditional heterogeneous catalysts. Reactions take place on the surfaces exposed to the reactants while the interface between the nanoparticles and the support maintains the high dispersion by preventing them from sintering. Studying such systems using state-of-the-art electron microscopy techniques can provide new knowledge of their working state and what processes govern their performance and stability. Aberration corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) can help unravel the structure of alloy materials. Specifically, PtxY and PtxGd alloy nanoparticles were studied. These catalysts are used for the oxygen reduction reaction (ORR) and show an activity which is more than 5 times higher compared to the best commercial Pt-based catalyst. Furthermore, these catalysts both retain more than 60% of their activity after accelerated aging tests. The electron microscopy investigations reveal the elemental distribution within the nanoparticles showing a core-shell type structure after ORR treatment. Figure 1 shows high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of platinum-yttrium nanoparticles in the form of core-shell structures. A core-shell structure is formed after the ORR chemical treatment, with an alloyed core embedded in a 1nm Pt-rich shell, due to the segregation of the Y from the first few atomic layers of the particle and its dissolution into the electrolyte. The Pt-rich shell prevents further dissolution of the rare earth metal protecting the alloyed core. Using Density Functional Theory (DFT), Pt-Hg and Pd-Hg were identified as promising candidates for the electrochemical production of hydrogen peroxide H2O2. The active surface was predicted to consist of reactive Pt or Pd atoms surrounded by more inert Hg atoms. Electrochemical measurements on the two catalysts have shown a performance exceeding the current state-of-the-art. Using Electron microscopy the structure and composition of the nanoparticle alloys has been determined and it confirms the theoretical predictions. In Pt-Hg nanoparticles, EDS studies has indicated the presence of both elements on single particles, although high-resolution imaging has shown particles where pure Pt was the only matching structure. In the case of Pd-Hg, a core-shell structure was found, with a pure Pd core and a Pd-Hg shell. Through atomic resolution STEM, the structure of the alloy in the shell of different particles was revealed, showing the formation of an ordered alloy structure. Studying deactivation mechanisms is an important discipline in catalysis. Environmental transmission electron microscopy (ETEM) provides a unique platform where catalysts can be studied in situ in a reactive environment. Using ETEM, the structure and morphology of catalysts and how the structures behave during deactivation can be investigated in an environment resembling that in a catalytic reactor in real time. Here the sintering of nanoparticles were studied. Model catalytic systems consisting of supported nickel, palladium, gold and platinum nanoparticles were studied in situ in the microscope and time-lapsed image sequences recorded. Analysis of these image sequences reveal that both particle migration and Ostwald ripening can take place. These studies reveal the underlying mechanisms governing the sintering process. The technique was also used to study the activation and deactivation processes of Ni-YSZ cermets used in solid oxide fuel cells where we have shown the reduction of single crystalline NiO to metallic Ni followed by the reoxidation to polycrystalline NiO. Electron microscopy provides a versatile platform for investigations of catalysts. Using various EM techniques, new insight into the active phase and deactivation processes of catalysts can be obtained. Figure 1
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- 2015
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174. Surface-charge-induced reversible phase transitions of Bi nanoparticles
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Jens-Oliver Müller, Robert Schlögl, Jakob Birkedal Wagner, Marc Georg Willinger, and Dang Sheng Su
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Phase transition ,Nanostructure ,Materials science ,Surface Properties ,Static Electricity ,Molecular Conformation ,Nanoparticle ,Nanotechnology ,Dielectric ,Phase Transition ,Biomaterials ,Metastability ,Materials Testing ,Electrochemistry ,General Materials Science ,Surface charge ,Particle Size ,High-resolution transmission electron microscopy ,General Chemistry ,Nanostructures ,Ferromagnetism ,Chemical physics ,Crystallization ,Bismuth ,Biotechnology - Abstract
Much of the interest in nanostructured materials originates from the challenge to find materials with properties that are substantially different from those of the same materials on a larger scale. New dielectric, [1] ferromagnetic, [2] catalytic, [3] and other properties are indeed being realized. Of particular interest are nanoparticles that exist in new structures, metastable structures, or known structures extended beyond the composition or stability ranges of ordinary bulk materials.
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- 2006
175. Phase segregation in AlInP shells on GaAs nanowires
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Werner Seifert, Niklas Sköld, Mats-Erik Pistol, Lars Samuelson, Tania Hernan, Gunnel Karlsson, and Jakob Birkedal Wagner
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Photoluminescence ,Morphology (linguistics) ,Condensed matter physics ,business.industry ,Chemistry ,Mechanical Engineering ,Diffusion ,Nanowire ,Shell (structure) ,Bioengineering ,General Chemistry ,Condensed Matter Physics ,Optics ,Phase (matter) ,Scanning transmission electron microscopy ,General Materials Science ,Facet ,business - Abstract
We have studied morphology and phase segregation of AlInP shells on GaAs nanowires. Photoluminescence measurements on single core−shell nanowires indicated variations in the shell composition, and phase segregation was confirmed by cross-sectional scanning transmission electron microscopy on 30 nm thin slices of the wires. It was discovered that Al-rich domains form in the 〈112〉 directions where two {110} shell facets meet during growth. We propose that the mechanism behind this phase segregation is a variation in the chemical potential along the circumference of the nanowire together with a difference in diffusion lengths for the different growth species. From the morphology of the core and the shell, we conclude that the side facet growth is temperature dependent forming {112} facets at low growth temperature and {110} facets at high growth temperature.
- Published
- 2006
176. Beaded Silicon Carbide Nanochains via the Carbothermal Reduction of Carbonaceous Silica Xerogel
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Ya-Juan Hao, Xiang-Yun Guo, Jakob Birkedal Wagner, Dang Sheng Su, and Guoqiang Jin
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Morphology (linguistics) ,Materials science ,Nanostructure ,Mechanical Engineering ,Inorganic chemistry ,Composite number ,Bioengineering ,General Chemistry ,Epoxy ,chemistry.chemical_compound ,chemistry ,Chemical engineering ,Mechanics of Materials ,Bromide ,Carbothermic reaction ,visual_art ,Ultimate tensile strength ,visual_art.visual_art_medium ,Silicon carbide ,General Materials Science ,Electrical and Electronic Engineering - Abstract
Novel silicon carbide nanostructures, beaded nanochains, are prepared from the carbothermal reduction of a carbonaceous silica xerogel with cetyltrimethylammonium bromide and lanthanum nitrate as additives. The nanochains consist of a stem with a diameter of about 50 nm and uniform beads with diameters of 100–200 nm. It is demonstrated that the tensile strength of an epoxy composite filled with the SiC nanochains improves significantly due to the unusual morphology of the nanochains.
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- 2006
177. Automated Structure Detection in HRTEM Images: An Example with Graphene
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Jakob Birkedal Wagner, Rasmus Larsen, Jacob S. Vestergaard, Thomas Willum Hansen, Anders Bjorholm Dahl, and Jens Kling
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Materials science ,Graphene ,law ,Nanotechnology ,High-resolution transmission electron microscopy ,Instrumentation ,law.invention - Abstract
Graphene, as the forefather of 2D-materials, attracts much attention due to its extraordinary properties like transparency, flexibility and outstanding high conductivity, together with a thickness of only one atom. The properties seem to be dependent on the atomic structure of graphene and therefore characterizations on the atomic level are of interest. High-resolution transmission electron microscopy (HRTEM) is a state-of-the-art method to characterize the atomic structure of materials. Due to the inherently low mass-thickness of graphene, the contrast levels in the recorded images are often challenging to interpret. In order to increase the signal-to-noise ratio of the images two routes can be pursued: 1) the exposure time can be increased; or 2) acquiring series of images and summarize them after alignment. Both methods have the disadvantage of summing images acquired over a certain period of time making it difficult to resolve dynamic processes or unstable structures. Tools that assist to get the maximum of information out of recorded images are therefore greatly appreciated.In order to get the most accurate results out of the structure detection, we have optimized the imaging conditions used for the FEI Titan ETEM with a monochromator and an objective-lens Cs-corrector. To reduce the knock-on damage of the carbon atoms in the graphene structure, the microscope was operated at 80kV. As this strongly increases the influence of the chromatic aberration of the lenses, the energy spread of the electron gun was reduced. Using the monochromator an energy spread of These images serve as a basis for the image analysis. Single-layer graphene with its regular honeycomb lattice is a perfect model structure to apply automated structure detection. By utilizing Fourier analysis the initial perfect hexagonal structure can easily be recognized. The recorded hexagonal tessellation reflects the unperturbed structure in the image. The centers of the C-hexagons are displayed as nodes. To segment the image into “pure” and “impure” regions, like areas with residual amorphous contamination or defects e.g. holes, a sliding window approach is used. The magnitude of the Fourier transformation within a window is compared to that of a perfect hexagonal tessellation. Areas where this relation exceeds a threshold are recognized as “impure” and a mask is created. As a result, the hexagonal tessellation overlays only the “pure” graphene structure in the image.As the real graphene structure is never perfect and undisturbed, at least at a length-scale of several nm, the model structure has to be adjusted to the real structure. At this point, the image quality plays a crucial role. The algorithm assumes that irregularities in the graphene can be explained by a deformation in the xy-plane. To model this, a set of tensor B-splines is employed, which is deformed by matching model grid points with the C-hexagon centers. Dependent on the Cs and defocus-settings during microscopy these centers appear either dark or bright. One ends up with a deformed hexagonal tessellation, which can easily be transformed into a honeycomb lattice with the C-atom positions included. As the microstructure is now available in the model, information like the C-C distance can be visualized as shown in fig. 2.Applying this method, the perfect graphene structure in recorded HRTEM-images can be determined fast and accurate over a wide length scale, and at the same time lattice deformations can be visualized. The method will be refined to facilitate the detection of larger defects like holes and the determination of the edge terminations.
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- 2013
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178. Adsorption of CO2 and coadsorption of H and CO2 on potassium-promoted Cu(115)
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Alessandro Baraldi, Jakob Birkedal Wagner, Giorgio Paolucci, Jens Onsgaard, Søren Vrønning Hoffmann, Amela Groso, P. Jan Godowski, Giovanni Comelli, Palle Møller, Onsgaard, J, Hoffmann, S. V., Moller, P., Godowski, P. J., Wagner, J. B., Paolucci, G., Baraldi, Alessandro, Comelli, Giovanni, and Groso, A.
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Potassium ,Inorganic chemistry ,Analytical chemistry ,chemistry.chemical_element ,Alkali metal ,Copper ,Atomic and Molecular Physics, and Optics ,Dissociation (chemistry) ,chemistry.chemical_compound ,Adsorption ,chemistry ,X-ray photoelectron spectroscopy ,Desorption ,Formate ,Physical and Theoretical Chemistry - Abstract
The influence of potassium, in the submonolayer regime, on the adsorption and coadsorption of CO2 and H on a stepped copper surface, Cu(115), has been studied by photoelectron spectroscopy, temperature-programmed desorption, and work-function measurements. Based on the fast recording of C 1s and O 1s core-level spectra, the uptake of CO2 on K/Cu(115) surfaces at 120 K has been followed in real time, and the different reaction products have been identified. The K 2p3/2 peak exhibits a chemical shift of -0.4 eV with CO2 saturation, the C 1s peaks of the CO3 and the CO species show shifts of -0.8 and -0.5 eV, respectively, and the C 1s peak of the physisorbed CO2 exhibits no shift. The effects of gradually heating the CO2/K/Cu(115) surface include the desorption of physisorbed CO2 at 143 K; the desorption of CO at 193 K; the ordering of the CO3 species, and subsequently the dissociation of the carbonate with desorption at 520-700 K. Formate, HCOO-, was synthesized by the coadsorption of H and CO2 on the K/Cu(115) surface at 125 K. Formate formed exclusively for potassium coverages of less than 0.4 monolayer, whereas both formate and carbonate were formed at higher coverages. The desorption of formate-derived CO2 took place in the temperature range 410-425 K and carbonate-derived CO2 desorbed at 645-660 K, depending on the potassium coverage.
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- 2003
179. Nickel oxide reduction studied by environmental TEM and in situ XRD
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Quentin Jeangros, Aïcha Hessler-Wyser, Rafal E. Dunin-Borkowski, Jakob Birkedal Wagner, Cécile Hébert, Thomas Willum Hansen, and Christian Danvad Damsgaard
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Diffraction ,In situ ,Materials science ,Nickel oxide ,Environmental Transmission Electron Microscope ,Non-blocking I/O ,Analytical chemistry ,Crystallite ,Mass spectrometry ,Instrumentation ,Diffractometer - Abstract
In situ reduction of an industrial NiO powder from JT BakerTM is performed under 1.3 mbar of H2 (2 mlN/min) in a differentially pumped FEI Titan environmental transmission electron microscope (ETEM) [3]. Images, diffraction patterns and electron energy loss spectra are acquired during different temperature ramps (300 to 600°C) to monitor the structural and chemical evolution. Similar experiments are performed in a PANalytical X'Pert PRO X-ray diffractometer equipped with an Anton Paar furnace and a mass spectrometer to complement and extrapolate ETEM data to higher pressure and flow conditions. Peak intensities, crystallite size, H2 and H2O signals are monitored.
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- 2012
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180. In situ environmental transmission electron microscopy investigation of core-shell supported co-catalyst system for optimized visible-light water splitting
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Jakob Birkedal Wagner, S. Dahl, Filippo Cavalca, Fabio Dionigi, Thomas Willum Hansen, and A.B. Laursen
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Core-shell ,In situ ,Materials science ,Analytical chemistry ,Catalysis ,Core shell ,Transmission electron microscopy ,TEM ,Water splitting ,Energy filtered transmission electron microscopy ,SDG 7 - Affordable and Clean Energy ,Photocatalysis ,Instrumentation ,Visible spectrum - Abstract
Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.
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- 2012
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181. Fast Electron – Gas Interactions in an Environmental TEM
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Thomas Willum Hansen and Jakob Birkedal Wagner
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Materials science ,Chemical physics ,Analytical chemistry ,Fermi gas ,Instrumentation - Abstract
Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.
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- 2012
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182. Organic light emitting diodes with environmentally and thermally stable doped graphene electrodes
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John Robertson, Jakob Birkedal Wagner, Piran R. Kidambi, Jens Kling, Stephan Hofmann, Arun Kuruvila, and Jens Meyer
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Materials science ,business.industry ,Graphene ,Doping ,Charge (physics) ,General Chemistry ,law.invention ,Metal ,law ,visual_art ,Electrode ,Materials Chemistry ,OLED ,visual_art.visual_art_medium ,Optoelectronics ,Thermal stability ,Doped graphene ,business - Abstract
We present a comparative study of the environmental and thermal stability of graphene charge transfer doping using molybdenum–trioxide (MoO3), vanadium–pentoxide (V2O5) and tungsten–trioxide (WO3). Our results show that all these metal oxides allow a strong and stable p-type doping of graphene, as well as functioning as effective hole-injection layers for highly efficient organic light emitting diodes.
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- 2014
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183. Atomic-resolution in situ transmission electron microscopy of a promoter of a heterogeneous catalyst
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Claus J. H. Jacobsen, Poul Hansen, Jakob Birkedal Wagner, Thomas Willum Hansen, Haldor F A Topsoe, and Søren Dahl
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Reaction mechanism ,Multidisciplinary ,biology ,Chemistry ,Stereochemistry ,chemistry.chemical_element ,Active site ,Heterogeneous catalysis ,Overlayer ,Catalysis ,Ruthenium ,Characterization (materials science) ,Ammonia production ,Chemical engineering ,biology.protein - Abstract
Insight into the location, state, and function of a promoter in heterogeneous catalysis was obtained through atomic-resolution in situ transmission electron microscopy. In the most active ruthenium catalyst for ammonia synthesis known so far, the barium promoter is shown to be located in two different phases in the catalyst. The increased activity is suggested to be related to a two-dimensional barium-oxygen overlayer on the ruthenium crystals. The possibility for conducting such studies for other reactions could add substantially to our current understanding of heterogeneous catalysis. Heterogeneous catalysis plays an increasingly important role in environmental protection processes, in fuel upgrading, and in providing the majority of the chemical building blocks required by contemporary society. Most heterogeneous catalysts of industrial importance are multicomponent materials that are designed by trial-and-error experimentation. Application of even the most sophisticated physical-chemical characterization techniques is usually not sufficient to obtain a complete understanding of the structure of the active site, the reaction mechanism and kinetics, the structural dynamics, and the specific roles of all catalyst components.
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- 2001
184. Dynamical Response of Catalytic Systems in a Cs Corrected Environmental Transmission Electron Microscope
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Chris Boothroyd, Marco Beleggia, Jakob Birkedal Wagner, Rafal E. Dunin-Borkowski, and Thomas Willum Hansen
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Conventional transmission electron microscope ,Materials science ,business.industry ,Environmental Transmission Electron Microscope ,Scanning transmission electron microscopy ,Optoelectronics ,Energy filtered transmission electron microscopy ,business ,Instrumentation ,Environmental scanning electron microscope ,Catalysis - Abstract
Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
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- 2010
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185. Dissociation of CO and formation of carbonate on a stepped, K-modified Cu(115) surface
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J. Onsgaard, Giovanni Comelli, P. Møller, Alessandro Baraldi, Giorgio Paolucci, Jakob Birkedal Wagner, P. J. Godowski, Søren Vrønning Hoffmann, Amela Groso, Onsgaard, J., Hoffmann, S. V., Godowski, P. J., Moeller, P., Wagner, J. B., Groso, A., Baraldi, Alessandro, Comelli, Giovanni, and Paolucci, G.
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Electron energy ,Annealing (metallurgy) ,Inorganic chemistry ,Analytical chemistry ,General Physics and Astronomy ,Synchrotron radiation ,chemistry.chemical_element ,Alkali metal ,Oxygen ,Dissociation (chemistry) ,chemistry.chemical_compound ,chemistry ,Valence band ,Carbonate ,Physical and Theoretical Chemistry - Abstract
It is demonstrated that CO dissociates on a potassium-modified Cu(115) surface at 125 K. The dissociation process is conditioned by both the presence of steps and an alkali metal coverage in the range of 0.4
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- 2000
186. The Titan Environmental Transmission Electron Microscope: Specifications, Considerations and First Results
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Jörg R. Jinschek, Rafal E. Dunin-Borkowski, Jakob Birkedal Wagner, and Thomas Willum Hansen
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Conventional transmission electron microscope ,business.industry ,Chemistry ,Ultra-high vacuum ,Low-voltage electron microscope ,Characterization (materials science) ,law.invention ,Optics ,law ,Environmental Transmission Electron Microscope ,Scanning transmission electron microscopy ,Energy filtered transmission electron microscopy ,Electron microscope ,business ,Instrumentation - Abstract
characterization techniques has increased dramatically. In electron microscopy, this demand constitutes an intrinsic challenge as the electron source requires high vacuum to function. Nevertheless, in the 1970’s, transmission electron microscopes (TEMs) were first adapted for use with gases [1]. Such machines are known as
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- 2009
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187. TEM Correlation between the Structural and Optical Properties Of Rotationally Twinned InP Nanowires
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Thomas Mårtensson, Lars Samuelson, David C. Bell, Johanna Trägårdh, Jakob Birkedal Wagner, Jiming Bao, and Federico Capasso
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Materials science ,business.industry ,Nanowire ,Optoelectronics ,business ,Instrumentation - Abstract
Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008
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- 2008
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188. The measurement of electrostatic potentials in core/shell GaN nanowires using off-axis electron holography
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R. Ciechonski, Takeshi Kasama, Jakob Birkedal Wagner, O. Kryliouk, and Sadegh Yazdi
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History ,Materials science ,Dopant ,business.industry ,Doping ,Nanowire ,Shell (structure) ,Nanotechnology ,Focused ion beam ,Electron holography ,Computer Science Applications ,Education ,Core (optical fiber) ,Optoelectronics ,Rectangular potential barrier ,business - Abstract
Core-shell GaN nanowires are expected to be building blocks of future light emitting devices. Here we apply off-axis electron holography to map the electrostatic potential distributions in such nanowires. To access the cross-section of selected individual nanowires, focused ion beam (FIB) milling is used. Furthermore, to assess the influence of FIB damage, the dopant potential measured from an intact NW is compared with a FIB prepared one. It is shown that in addition to the built-in potential between the p-type shell and unintentionally n-type under-layer there is a potential barrier between the core and under-layer which are both unintentionally n-type doped.
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- 2013
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189. Vibrational mode and dielectric function spectra of BGaP probed by Raman scattering and spectroscopic ellipsometry
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S. Liebich, Kerstin Volz, M. Baeumler, S. Rogowsky, M. Wolfer, Jakob Birkedal Wagner, R. Ostendorf, Wolfgang Stolz, Bernardette Kunert, Lutz Kirste, and Publica
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Materials science ,Band gap ,Scattering ,Doping ,Analytical chemistry ,General Physics and Astronomy ,chemistry.chemical_element ,Isotopes of boron ,symbols.namesake ,chemistry ,Ellipsometry ,symbols ,Boron ,Raman spectroscopy ,Raman scattering - Abstract
The influence of boron incorporation in Bx Ga1-xP (0
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- 2011
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190. Ultrahigh resolution focused electron beam induced processing: the effect of substrate thickness
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W. F. van Dorp, Jakob Birkedal Wagner, Armin Gölzhäuser, Ivan Lazic, André Beyer, Thomas Willum Hansen, and Cornelis W. Hagen
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Yield (engineering) ,Materials science ,Mechanical Engineering ,Analytical chemistry ,chemistry.chemical_element ,Bioengineering ,General Chemistry ,Chemical vapor deposition ,Substrate (electronics) ,Electron ,Secondary electrons ,chemistry ,Mechanics of Materials ,Cathode ray ,General Materials Science ,Growth rate ,Electrical and Electronic Engineering ,Carbon - Abstract
It is often suggested that the growth in focused electron beam induced processing (FEBIP) is caused not only by primary electrons, but also (and even predominantly) by secondary electrons (SEs). If that is true, the growth rate for FEBIP can be changed by modifying the SE yield. Results from our Monte Carlo simulations show that the SE yield changes strongly with substrate thickness for thicknesses below the SE escape depth. However, our experimental results show that the growth rate is independent of the substrate thickness. Deposits with an average size of about 3 nm were written on 1 and 9 nm thick carbon substrates. The apparent contradiction between simulation and experiment is explained by simulating the SE emission from a carbon substrate with platinum deposits on the surface. It appears that the SE emission is dominated by the deposits rather than the carbon substrate, even for deposits as small as 0.32 nm(3).
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- 2011
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191. Atomic resolution imaging ofin situInAs nanowire dissolution at elevated temperature
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Rafal E. Dunin-Borkowski, Jakob Birkedal Wagner, Chris Boothroyd, Robert S. Pennington, and Joerg R. Jinschek
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In situ ,History ,Materials science ,Oxide ,Nanowire ,chemistry.chemical_element ,Nanotechnology ,Computer Science Applications ,Education ,Catalysis ,chemistry.chemical_compound ,chemistry ,Environmental Transmission Electron Microscope ,Crystallite ,Dissolution ,Indium - Abstract
We present a preliminary study of the in situ heating of InAs nanowires in a gaseous environment in an environmental transmission electron microscope. Nanowire dissolution, accompanied by dynamic reshaping of crystalline Au-containing catalyst particles at the ends of the wires, is observed, accompanied by indium oxide crystallite formation nearby.
- Published
- 2010
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192. In Situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Titan ETEM
- Author
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Antonin Faes, Quentin Jeangros, Jakob Birkedal Wagner, Aïcha Hessler-Wyser, Jane Van herle, Annabelle Brisse, and Dunin-Borkowski, Rafal E.
- Abstract
not Available.
- Published
- 2009
- Full Text
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193. Understanding the 3D structure of \mathrm {GaAs\langle 111\rangle B} nanowires
- Author
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Kimberly A. Dick, Knut Deppert, Lisa Karlsson, Jan-Olle Malm, Lars Samuelson, Jakob Birkedal Wagner, and L. Reine Wallenberg
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Materials science ,Condensed matter physics ,Mechanical Engineering ,Nanowire ,Bioengineering ,Crystal growth ,General Chemistry ,Epitaxy ,Crystallography ,Mechanics of Materials ,Transmission electron microscopy ,Microscopy ,General Materials Science ,Lamellar structure ,Electrical and Electronic Engineering ,High-resolution transmission electron microscopy ,Crystal twinning - Abstract
The effects of lamellar twinning in epitaxial particle-assisted GaAs B nanowires are investigated in an extensive high resolution electron microscopy (HRTEM) study of the low index zones , , and . As these directions are non-parallel to the (-1-1-1) twin planes we find that the twin segments exhibit two different zone axes as a consequence of twinning. In the first three cases the alternative zones were found to be and . These findings are supported by a comparison of experimental HRTEM images and multi-slice simulations along with fast Fourier transform mapping. From the appearance of non-overlapping regions we conclude that the nanowires are bound by {111} facets only. The twin formation and the development of the stable side facets are discussed (Less)
- Published
- 2007
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194. Measurements of the band gap of wurtzite InAs1−xPx nanowires using photocurrent spectroscopy
- Author
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Johanna Trägårdh, Lars Samuelson, Dan Hessman, Jakob Birkedal Wagner, and Ann Persson
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Photocurrent ,Materials science ,Condensed matter physics ,Band gap ,Photoconductivity ,Nanowire ,General Physics and Astronomy ,Direct and indirect band gaps ,Spectroscopy ,Semimetal ,Wurtzite crystal structure - Abstract
We report measurements of the band gap of InAs1−xPx nanowires having wurtzite crystal structure as a function of the composition for 0.14
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- 2007
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- View/download PDF
195. Strain mapping in free-standing heterostructured wurtzite InAs/InP nanowires
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Paul Håkansson, Lars Samuelson, Magnus Larsson, Mathias Wallin, L. Reine Wallenberg, Linus Fröberg, and Jakob Birkedal Wagner
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Materials science ,Condensed Matter::Other ,business.industry ,Mechanical Engineering ,Nanowire ,Physics::Optics ,Bioengineering ,Strain mapping ,General Chemistry ,Crystal structure ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter::Materials Science ,Crystallography ,Mechanics of Materials ,Transmission electron microscopy ,Lattice (order) ,Optoelectronics ,General Materials Science ,Electrical and Electronic Engineering ,business ,High-resolution transmission electron microscopy ,Anisotropy ,Wurtzite crystal structure - Abstract
The strain distribution in heterostructured wurtzite InAs/InP nanowires is measured by a peak finding technique using high resolution transmission electron microscopy images. We find that nanowires with a diameter of about 20 nm show a 10 nm strained area over the InAs/InP interface and the rest of the wire has a relaxed lattice structure. The lattice parameters and elastic properties for the wurtzite structure of InAs and InP are calculated and a nanowire interface is simulated using finite element calculations. Both the method and the experimental results are validated using a combination of finite element calculations and image simulations.
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- 2006
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196. Nanowire-based multiple quantum dot memory
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Claes Thelander, Jakob Birkedal Wagner, Linus Fröberg, Lars Samuelson, and Henrik Nilsson
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Materials science ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,business.industry ,Quantum wire ,Multiple quantum ,Nanowire ,Biasing ,Hysteresis ,Stack (abstract data type) ,Quantum dot ,Quantum dot laser ,Optoelectronics ,business - Abstract
The authors propose and demonstrate an alternative memory concept in which a storage island is connected to a nanowire containing a stack of nine InAs quantum dots, each separated by thin InP tunnel barriers. Transport through the quantum dot structure is suppressed for a particular biasing window due to misalignment of the energy levels. This leads to hysteresis in the charging/discharging of the storage island. The memory operates for temperatures up to around 150K and has write times down to at least 15ns. A comparison is made to a nanowire memory based on a single, thick InP barrier.
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- 2006
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197. Photoluminescence and excitation spectroscopy in heavily dopedn- andp-type silicon
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Jakob Birkedal Wagner
- Subjects
Materials science ,Photoluminescence ,Silicon ,business.industry ,Band gap ,Doping ,chemistry.chemical_element ,Electron ,Spectral line ,chemistry ,Optoelectronics ,Photoluminescence excitation ,Absorption (logic) ,Atomic physics ,business - Abstract
Low-temperature photoluminescence and excitation spectroscopy measurements on heavily doped (up to 4\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) $n$- and $p$-type silicon are reported. From the luminescence spectra values for the optical and the reduced band gap are deduced and compared with theoretical calculations. The shrinkage of the reduced band gap follows an ${n}^{\frac{1}{3}}$ law for carrier concentrations $n$ above the critical Mott density. Both $n$- and $p$-type samples show an identical shift of the reduced gap, whereas the shift of the optical gap is different due to the different density-of-states masses for electrons and holes. From photoluminescence excitation spectra the position of the optical gap is determined independently. A good agreement of the data obtained by these selective absorption measurements with the results from conventional luminescence spectra is found.
- Published
- 1984
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198. Effect of barrier configuration on excitonic recombination in Ga0.47In0.53As/Al0.48In0.52As multi quantum well structures
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Jakob Birkedal Wagner, K. H. Ploog, Wolfgang Stolz, and Publica
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Photoluminescence ,Condensed Matter::Other ,Chemistry ,Superlattice ,Rekombination(exzitonisch) ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Photolumineszenzspektroskopie ,Inorganic Chemistry ,Condensed Matter::Materials Science ,symbols.namesake ,Stokes shift ,Materials Chemistry ,symbols ,Photoluminescence excitation ,Emission spectrum ,Atomic physics ,Ternary operation ,Spectroscopy ,Quantum well - Abstract
Photoluminescence and photoluminescence excitation spectroscopy on Ga0.47In0.53As multi quantum wells confined either by homogeneous ternary Al0.48In0.52As barriers or by Ga0.47In0.53As/Al0.48In0.52As short-period superlattice (SPS) barriers show that the confinement by SPS barriers improves the edge luminescence significantly. The spectral width of the free-exciton absorption and the low-temperature emission peak as well as the Stokes shift between emission and excitation spectra are reduced as compared to samples clad by homogeneous ternary Al0.48In0.52As barriers. The dominant low-temperature emission line in the SPS-clad Ga0.47In0.53As multi quantum wells is assigned to intrinsic excitonic recombination. The small Stokes shift of the excitonic luminescence line can be explained by intrinsic relaxation processes caused by monolayer fluctuation of the well width and by statistical fluctuation of the composition of the ternary Ga0.47In0.53As well material. (IAF)
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- 1987
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199. Identification of electronic and vibronic states of the deep indium-related defect in quenched Si: In by excitation spectroscopy
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Jakob Birkedal Wagner and R. Sauer
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Excitation spectroscopy ,Materials science ,chemistry ,Atomic electron transition ,Excited state ,chemistry.chemical_element ,Vibronic spectroscopy ,Photoluminescence excitation ,Atomic physics ,Luminescence ,Indium ,Electronic states - Abstract
High-sensitivity photoluminescence excitation (PLE) measurements were performed on the deep In-related isoelectronic defect in quenched Si: In responsible for the $P$,$Q$,$R$ luminescence lines. A complex set of excited states was obtained. Combining the PLE and conventional PL results, we identify three electronic states 3.8, 11.4, and 36.4 meV above the $P$-line initial state. These do not form a hydrogenlike series. Localized vibrations are identified to give rise to one-and two-phonon, Stokes, and anti-Stokes sidebands of the electronic transitions.
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- 1983
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200. In-Situ Reduction of Promoted Cobalt Oxide Supported on Alumina by Environmental Transmission Electron Microscopy
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Anders Holmen, Thomas Willum Hansen, Erling Rytter, Roya Dehghan, Øyvind Borg, Jakob Birkedal Wagner, and John C. Walmsley
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Chemistry(all) ,Chemistry ,Analytical chemistry ,chemistry.chemical_element ,General Chemistry ,Catalysis ,Transmission electron microscopy ,Environmental Transmission Electron Microscope ,Scanning transmission electron microscopy ,Energy filtered transmission electron microscopy ,Electron beam-induced deposition ,High-resolution transmission electron microscopy ,Cobalt ,Cobalt oxide - Abstract
The Author(s) 2011. This article is published with open access at Springerlink.com Abstract Reduction of 12wt.%Co/0.5wt.%Re/a-Al2O3 Fischer-Tropsch catalyst has been studied in-situ in an environmental transmission electron microscope. Reduction of Co3O4 to metallic cobalt was observed dynamically at 360 C under 3.4 mbar H2. Structural and morphological changes were observed by high resolution transmission electron microscopy and scanning transmission electron microscopy imaging. The cobalt particles were mainly face centred cubic while some hexagonal close packed particles were also found. Reoxidation of the sample upon cooling to room temperature, still under flowing H2, underlines the reactivity of the nanoparticles and the importance of con- trolling the gas composition and specimen temperature during this type of experiment. Similar behaviour was observed for a non-promoted catalyst. Imaging and analysis of the promoted sample before and after reduction indicated a uniform distribution of the promoter.
- Full Text
- View/download PDF
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